Interpretive Summary: Understanding the possible effects of global climate change on agricultural production is an important concern for maintaining the future food supply. In particular, the anticipation of increasing concentrations of atmospheric carbon dioxide (CO2) has led to many studies on whole plant response to elevated levels of CO2. There is far less information available on the effects of elevated CO2 on plant growth under salinity stress than under non-saline conditions, yet it is under stress that CO2 effects may be most significant. To study the interactive effects of salinity and elevated CO2 on growth of tomato, we grew plants two growth chambers. The CO2 level in one chamber was ambient and in the second, twice ambient. In each chamber the plants were subjected to different levels of salinity ranging from a nonsaline control treatment (plants grew luxuriously) to a highly saline treatment (plants were stunted). We determined plant growth, root to shoot ratio, water use, and leaf chloride accumulation. We used these measurements to compare two different methods of evaluating salt tolerance. From our analysis, we found that the salinity stress index was more stable and less dependent on environmental variables, such as fluctuating CO2 levels, than the rootzone salinity procedure.

Technical Abstract:
Although there is consensus that water use efficiency increases at elevated concentrations of CO2, there are few studies on the interacting effects of elevated CO2 on plant salt tolerance. The objectives of this study were, (1) to determine the effect of ambient and twice ambient concentrations of CO2 on tomato (Lycopersicon esculentum Mill.) responses to salinity; (2) to compare the salt tolerance threshold values based on the classic root zone salinity, or ion flux to the shoot, a measure recently defined as Salinity Stress Index (SSI). For all salinities, the water use of the twice ambient CO2 treatment was significantly reduced. The effect of twice ambient CO2 was to increase the root zone salinity threshold value from 32 to 51 mmol dm/3 Cl. The threshold SSI value of 1.05 mmol Cl per g shoot DW for the twice ambient CO2 treatment was almost identical to that of the ambient treatment and to those previously obtained when plant growth was modulated by root temperature (SSI = 1.19 and 1.10 at 25 and 18 deg C, respectively and photosynthetic photon flux density (PPFD) (SSE = 0.97 and 1.10 at 400 and 600 umol/m2/s PPFD respectively. The twice ambient CO2 treatment showed a slightly lower root/shoot ratio (0.138 + or - 0.001) than the ambient CO2 treatment (0.156 + or - 0.014). Consistent with the predictions of the SSI, leaf chloride per plant and leaf chloride concentration showed significant reduction for the twice ambient CO2 treatment which follows from the supposition that water and salt uptake are linked. Based on the SSI, it was shown that the intrinsic salt tolerance of tomato is invariant to an increase in atmospheric CO2 as has been previously shown for root temperature and solar radiation, while at the same time, the root zone salinity threshold value is dependent on environmental factors.